In a sample observation using an electron microscope, a so-called “in-situ observation,” in which a sample in a gas atmosphere and a reaction process of the gas are dynamically observed in situ, is performed in some cases. For example, as for a fuel cell catalyst, when the catalyst is exposed to gas, precious metal particles move on a support, and particle growth occurs. The particle growth is observed in situ to perform a catalyst deterioration analysis.
To perform the observation of this kind, it is necessary to divide a main space in a column held in a high vacuum state from a space around the sample held in a low vacuum state by gas. A method therefor is roughly classified into a diaphragm type and a differential pumping type. As one of techniques relating to the former type, JP 2003-187735 A describes a sample holder sealing a sample in a gas atmosphere, for example. This sample holder includes a sample mounting portion provided with an opening adapted to let an electron beam pass therethrough, a heater wire (a sample heating device) stretched to cross the opening, a diaphragm adapted to isolate the sample mounting portion from a sample chamber (vacuum), and a gas inlet tube (a gas inlet device) adapted to introduce gas into a sample mounting space formed by the diaphragm.